Hydrocarbon combustion in diesel engines, stationary gas turbines, and other systems generates exhaust gas that must be treated to remove nitrogen oxides (NOx), including NO, NO2, and N2O. The exhaust generated in lean-burn engines is generally oxidative, and the NOx needs to be reduced selectively with a heterogeneous catalyst and a reductant, which is typically ammonia or a short-chain hydrocarbon. The process, known as selective catalytic reduction (SCR), has been thoroughly investigated.
Many known SCR catalysts utilize a transition metal (e.g., Cu, Fe, or V) coated on a high-porosity support, such as alumina or a zeolite. Zeolites are a well-known variety of molecular sieves that are mostly regular frameworks built from TO4 tetrahedra, in which T is typically silicon, aluminum, or phosphorus.
Manganese oxide octahedral molecular sieves (“OMS”) are also known. As the name suggests, octahedral units combine to make the overall structure, which is characterized by one-dimensional channels. Some manganese oxide OMS occur in nature, including hollandites (hollandite, cryptomelane, manjiroite, coronadite) and the poorly crystalline todorokites. Manganese oxide OMS have also been synthesized (see, e.g., U.S. Pat. Nos. 5,340,562; 5,523,509; 5,545,393; 5,578,282; 5,635,155; and 5,702,674 and R. DeGuzman et al., Chem. Mater. 6 (1994) 815). In some cases, some of the manganese in the framework of an OMS can be substituted with other metal ions. This is usually accomplished by doping other ions in the process used to make the manganese oxide OMS. For instance, U.S. Pat. No. 5,702,674 teaches to substitute Fe, Cu, Mo, Zn, La, or other metals for Mn in the framework of a manganese oxide OMS. As this reference teaches, manganese oxide OMS are potentially useful for reducing nitric oxide with ammonia, although relatively little is known about their use for an SCR process.
Natural manganese ores (hollandite, cryptomelane) have been used for low-temperature SCR of nitrogen oxides with ammonia (see, e.g., Tae Sung Park et al., Ind. Eng. Chem. Res. 40 (2001) 4491).
Manganese oxide OMS catalysts have some drawbacks. For instance, the OMS catalysts can be thermally unstable such that NOx conversion can diminish rapidly as the catalyst ages or is exposed to high temperatures. Moreover, the low-temperature NOx conversion, i.e., at temperatures from 100° C. to 250° C., is typically less than desirable. This is important because lean-burn engines—which are characterized by air/fuel ratios >15, typically 19-50—generate considerable NOx immediately after start-up when the exhaust gas temperature is at its lowest.
More recently, other metals have been suggested for use as dopants for manganese oxide OMS. For instance, vanadium-doped cryptomelane-type manganese oxides (V-OMS-2) have been synthesized and used for low-temperature SCR or NO by ammonia (NH3-SCR) (see Liang Sun et al., Appl. Catal. A 393 (2011) 323). Similarly, Chao Wang et al. describe hollandite-type manganese oxides with K+ or H+ in the tunnels and their use for low-temperature NH3-SCR (Appl. Catal. B 101 (2011) 598.
Rare-earth metals, including cerium oxides, have been used as components of SCR catalysts (see, e.g., U.S. Pat. Nos. 4,695,437, 4,782,039, and 8,091,351 and references cited therein). For instance, WO 2004/002611 teaches an NH3-SCR catalyst comprising a ceria-doped zeolite. However, cerium oxides do not appear to have been used to modify manganese oxide OMS for use in an SCR process.
The industry would benefit from improved SCR catalysts, particularly low-temperature NH3-SCR catalysts.